The following applications filed concurrently herewith are not necessarily related to the present application, but are incorporated by reference herein in their entirety: “Methods to Monitor System Sensor and Actuator Health and Performance” (U.S. patent application Ser. No. 11/700,735, filed simultaneously with the effective filing date of the present application; “Systems for Monitoring Sensor and Actuator Health and Performance” (U.S. patent application Ser. No. 11/700,396, filed simultaneously with the effective filing date of the present application; and “Methods for Managing Flow Control Valves in Process Systems” (U.S. patent application Ser. No. 11/700,533, filed simultaneously with the effective filing date of the present application.
Process plants, such as those located at a modem hydrocarbon production site, consist of many different pieces of equipment, each of which can be linked together to form a total process, or sub-units, such as various unit operations and sub-processes. These can be controlled using a network of control loops to achieve a particular end result for the process or a particular unit operation. Each control loop usually controls a particular process variable such as the flow rate of a material. The objective of the loop is to keep the process variable within a required operating range, usually at a desired set-point, to ensure the targeted process result. Each loop experiences, via external sources, or sometimes from internal creation by the loop itself, disturbances that cause the process variable to move away from the set-point. Additionally, interaction with other loops in the process network can cause disturbances that can affect the process variable.
To assist control loops, sensors gather process variable data. For example, on a drilling rig, the drilling mud fluid must be provided within specific compositional and flow stream parameters. Sensors can monitor the flow rate of the mud, pressure, density, and other process variables, and this information is typically fed back to a control unit which operates individual control loops for each of the controlled variables.
Sometimes, a particular process variable is not easily measured. In those instances process variable “observers” are used to provide the indirect measurement, or inference, of a particular process variable. See U.S. patent application Ser. No. 11/121,144 as Publication Number US 2006/0235627, filed May 3, 2005, entitled “Methods and Systems for Estimating Density of a Material in a Mixing Process”, to Jason D. Dykstra and Justin A. Borgstadt, for an example of a density observer, which is hereby incorporated by reference in its entirety.
Within a control unit, each control loop normally has its own controller which has a specifically-tailored algorithm to achieve a particular process variable control function. The controller usually processes the sensor and set-point information and then decides what must be done to get the process variable back to set-point when a disturbance occurs. This decision takes the form of a control signal. Once a control signal is generated, a “final control element” must implement the signal from the controller and apply it to the physical process. Examples of common final control elements for fluid applications are control valves, speed controlled-pumps, and speed-controlled compressors. Common final control elements for solids-handling applications are variable-speed drives for material flow control devices such as screw conveyors, belt feeders, slide gate valves, and rotary feeder valves.
In particular, flow control valves are very common in process control applications. They act to increase or decrease the flow of a fluid or solid, in response to a control signal, where a fluid can be a liquid, gas, or vapor, or various combinations thereof. A flow control valve generally consists of at least a valve body, a moveable member within the body for adjusting the open area for flow, a valve-to-actuator linkage, and an actuator.
Actuators are powered devices used by an automatic controller to convert a control signal into movement. Actuators are typically powered pneumatically, hydraulically, electrically, or mechanically. For flow control valves, actuators convert the control signal to a physical action via the control valve linkage. Such linkages, being mechanical devices, are subject to wear over time, resulting in many cases, as looseness, or mechanical slack. Such slack in actuator-to-valve connections creates a phenomena called “valve slop”. Thus, when the actuator moves the linkage in one direction based on a control signal, there is no opening or closing of the valve until the linkage moves far enough to “take-up” the slack caused by wear.
To illustrate, FIG. 3 shows an end view of an example rotary-actuated valve shaft 350 as a component of a rotary control valve. The valve was initially in fully closed 351 position 1. The valve actuator was then sent a signal to make a 45 degree rotation 354 to achieve desired position 2 shown as 353. Assuming the actuator-valve assembly is worn and has mechanical linkage slack, the actuator will have to first move to take up the “slack” before the shaft actually begins to turn. If the slack in the actuator linkage is equivalent to 45 degrees of rotation, shown as 356, then the actuator will need to signal 90 degrees of rotation shown as 352 to achieve 45 degrees of rotation for desired position 2. The span of unacted-upon control signal, prior to the initiation of movement, is a component of what is referred to as control valve “dead band.” In process control, the term “dead band” is defined as the range through which an input signal can be varied, upon reversal of direction, without initiating an observable change in the output signal. Said differently, dead band results from a temporary discontinuity between the input and output of a device when the input to the device changes direction.
FIG. 4 graphically illustrates the dead band concept as applied to flow control valves. The controller signal (“CS”) is the input to the valve assembly (to move the actuator), shown as the horizontal axis 410. The valve stem displacement “D”, which can be either linear or rotary, is shown on the vertical axis 400, which usually is expressed as “% OPEN” as noted in 440, where % OPEN is D divided by total available D times 100. For vertically-actuated valves, it can also be distance traveled. For rotary valves, it can also be expressed as % ROTATION, or in units of degrees or radians. Dead band is typically expressed as a percent of the input control signal span. Dead band at a first time period (T=1) is shown as span 420. Thus, as control signal is increased from X to Y, no displacement of the valve stem or shaft occurs due to the slack in the actuator-to-valve linkage, as previously described. Then, as CS goes to Z, the displacement occurs as displacement 440. However, any time the controller output reverses direction, the controller signal must again pass through the dead band. This is shown in the movement from Z in the reverse direction of CS to point XX wherein no displacement occurs (e.g. valve % OPEN remains constant).
Again referring to FIG. 4, at a second time period (T=2), it is assumed that the valve linkage has worn and developed slack. This is reflected in that the dead band has increased as shown as span 430 at T=2. Thus, the CS needs to increase from X to YY to overcome the dead band.
In FIG. 5, the flow through a control valve is depicted with relation to the displacement, D, of the control valve vertical stem or rotary shaft. It can be typified as either linear 500 or non-linear 510. It can be expressed as volumetric flow, shown as right-hand vertical axis 530, or as percent of total available flow through the valve, shown as left-hand axis 520. In one form, the valve coefficient, Cv, is the ratio of the flow rate through the valve over the displacement of the valve. Cv is dependent on a number of factors, including the condition of the regulating elements of the valve and associated components, such as the valve seat. For example, in a gate valve, the edge of the gate can be worn-away by the abrasive action of fluids or solids. Wear over time can cause Cv to change. Control algorithms often utilize the new, unused, as-installed Cv values as parameters, since they interact with control algorithm factors such as gain. For liquid applications, Cv is often expressed as the number of gallons per minute of 60° F. water that will flow through a particular valve with a one pound per square inch pressure drop.
One equation, as a non-limiting example of a flow model for a control valve, which relates flow through a valve to the valve coefficient is:Q is proportional to Cv×ƒ(l)where Q is the flow rate and ƒ(l) is the function which relates the % of flow to the displacement of the valve stem or shaft. For a linear-acting valve, ƒ(l)=1. Other flow control valve models are known to one skilled in the art of automatic flow control.
In FIG. 6, one effect of dead band on process control is illustrated. FIG. 6 shows changes in control signal where a valve has a dead band as shown in 620. Four small steps of change in control signal 600 are made from Time=0 to Time=A, before the dead band is exceeded on the fifth step, finally resulting in a change in valve displacement 610 (and the concomitant change in material flow rate). The pattern is repeated to Time=B and Time=C. However, the process controller then begins to send only 3 steps of change. For the first three steps from Time C, the valve shaft is displaced. But upon reversal of direction in signal, no displacement occurs, through to Time=D, because the dead band is not exceeded. The process controller is calling for changes in the process variable of flow, but because the dead band is so great in relation to the finer calls of control, the calls are never acted on. The net result is precision and responsiveness in overall process control is lost. Further, as wear or adjustments occur over time, the dead band increases, and process control gets increasingly less effective.
As a further source of error in digital signal control systems, FIG. 7 shows the error effect of quantization as analog signals are converted to digital signals (“A/D” conversion). For example, the signaled flow rate from a flow meter can be a 5 to 20 milliamp analog electrical signal 700, which is a continuous range of values. After quantization, the signal is now represented by a relatively small set of discrete symbols or integer values shown as digitized signal 710. Quantization can introduce error into signal processing because signal definition is lost between the different digital intervals, as shown in 720. Thus, as the true analog signal increase from 720A to just under 720B, the converted digital signal value remains the same. This amount of signal definition is thus lost. Because most modern control systems utilize A/D converters, further error is thus often introduced into control algorithms.
Because of the wear over time of mechanical actuator-to-valve linkages and flow-regulating elements of control valves, there is a need for accurate determination, compensation, and management of the effects of such wear in real time during process operations. More particularly, there is a need to reduce the uncertainty associated with hydrocarbon production operations.